1. Field of the Invention
The present invention relates to a process for preparing polyarylene sulfides and, more particularly, to a process for preparing polyarylene sulfides having a higher molecular weight and a high quality with high efficiency and stability. This process can dehydrate raw materials for polymerization at low temperatures and in a short period of time prior to polycondensation. Furthermore, this process can effectively prevent a production of impurities due to decomposition of the raw materials for polymerization. Thus this process is extremely advantageous for preparing the polyarylene sulfides.
2. Description of Related Art
Polyarylene sulfides such as polyphenylene sulfide are resins that are thermoplastic yet partially thermoset, and have excellent properties as engineering plastics, such as resistance to chemicals, mechanical properties over a wide range of temperatures, and resistance to thermal rigidity.
It is known that the polyarylene sulfides such as polyphenylene sulfide are generally prepared by polycondensation of a dihalogen aromatic compound with an alkali metal sulfide in an organic polar solvent. For example, polyphenylene sulfide is prepared by polycondensation of p-dichlorobenzene with sodium sulfide in an organic polar solvent, as disclosed in U.S. Pat. No. 3919177 (counterpart to Japanese Patent Publication No. 12,240/1977).
In order to provide polyarylene sulfides such as polyphenylene sulfide having excellent properties, it is required that their molecular weights become enlarged. Towards this end, a process has been proposed in which the water content in the reaction solution for polymerization is controlled. This process requires the step of dehydrating a hydrous alkali metal sulfide, before the polycondensation, because the alkali metal sulfides are usually available as a hydrate. This dehydration step has usually been carried out by azeotropically distilling off water in the presence of an organic polar solvent. Conventional dehydration steps as disclosed, for example, in Japanese Patent Publication (laid-open) Nos. 98,133/1984 and 7,332/1986 and U.S. Pat. Nos. 4,368,321 and 4,371,671 are carried out at atmospheric pressure using a distilling tower or rectification tower of the usual type. Thus, they suffer the disadvantages that, although heating at high temperatures is required for sufficiently dehydrating raw materials for polymerization, distillation at atmospheric pressure while heating at high temperatures causes solvents and the alkali metal sulfide to be decomposed, thereby leading to a poor quality of the resulting polymer and that the heating at a relatively low temperature as high as 150.degree. C. is unlikely to cause decomposition, but it takes a long dehydration time, thus lessening a production efficiency to a remarkably low extent.
On the other hand, various proposals have heretofore been made to provide the polyarylene sulfides with a higher molecular weight. For example, U.S. Pat. No. 4,038,263 discloses a process for preparing polyphenylene sulfide having a higher molecular weight by polymerizing p-dihalobenzene with an alkali metal sulfide in the presence of a lithium halide in an organic amide. This process contains the dehydration step in which a single distillation method is adopted for the dehydration step, for example, under a stream of nitrogen, however, there remains in a system undehydrated water in an amount higher than monohydrate per mole of the metal sulfide as a source of sulfur. The remaining water content impairs the action of lithium halide which should function as a polymerization aid although the lithium halide is added for the purpose to render the molecular weight of the resulting polyphenylene sulfide larger. As a result, the polyphenylene sulfide is not said to have a sufficient molecular weight.
Japanese Patent Publication (laid-open) No. 217,727/1984 discloses a process for preparing polyphenylene sulfide by polymerizing a polyhalogen aromatic compound with a mixture of 0.1% to 15% by weight of a substantially anhydrous sodium sulfite with 99.9% to 85% by weight of sodium sulfide in an organic polar solvent and under a condition where the water content present in the polymerization system is lower than 0.3 mole per mole of the sodium sulfide. However, this process is not sufficient because a molecular weight of the resulting polyphenylene sulfide is not high enough and a sulfur source to be used should be pre-processed to bring its water content to a given low level.
Japanese Patent Publication (laid-open) Nos. 22,926/1984 and 109,523/1984 propose processes in which dihalogen aromatic compounds are reacted with a substantially anhydrous metal sulfide and metal carbonate in the presence of a minute water content. These processes, however, produce a large quantity of by-products such as oligomers, and the objective products in lesser yields. Furthermore, they require a longer reaction time to make their molecular weights higher so that they are industrially disadvantageous.
A process for preparing polyphenylene sulfide is disclosed in Japanese Patent Publication No. 98,133/1984, which involves reacting p-dihalobenzene with a source of sulfur in a polar solvent of an organic amide type while the water content in the reaction system is still retained in the range from 0.3 to 0.95 mole per mole of the sulfur source. This process can provide polyphenylene sulfide which is relatively stable against energy such as heat, ultraviolet rays and the like, however, it is not said to have a sufficiently high molecular weight.